296 EETOET UNITED STATES GEOLOGICAL SURVEY. 



pressions or the cardinal teeth. Some of the casts, however, show that 

 there was a posterior lateral tooth in one or both valves, rather elon- 

 gated, parallel to the posterior dorsal margin, and a much shorter ante- 

 rior lateral close to the beak. Of course it will be impossible to deter- 

 mine the generic characters of these shells until more satisfactory speci- 

 mens can be obtained, and it is only provisionally that the species is 

 now referred to the genus Cyrena. The specific name is given in honor 

 of Mr. William H. Holmes, the artist of the Survey, who discovered the 

 type-specimens. 



"Locality and position. — On Ralston [Van Bibber?] Creek, three or four 

 miles north of Golden City, Colo., from beds supposed to hold a position 

 from 400 to 500 feet above the beds of coal mined at Golden City. 

 Probably of Tertiary age. I think Dr. Peale found the same species 

 farther south, between Golden and Colorado Springs, not far from the 

 latter. No other fossils were found associated with it." 



Figs. 5 a and b, plate x, represent what is understood to be the form 

 referred to in the closing paragraph quoted above as having been col- 

 lected by Dr. Peale in the neighborhood of Colorado Springs. The spec- 

 imens are all imbedded in hard rock, but by breaking many of them 

 they are found to have essentially the hinge structure of Mactra, but 

 the minor details of parts of the hinge have not been satisfactorily ascer- 

 tained on account of the brittlcness of the fragile shells and the com- 

 pactness of the imbedding rock. 



Figure 4 c, on plate 0, is drawn from Mr. Meek's type. Figures 4 a 

 and b of the same plate are of associated examples. An examination of 

 a number of examples obtained by myself at the typical locality, four 

 miles north of Golden City, leaves little or no doubt in my mind that the 

 true Cyrena f holmesi also has a hinge in all respects like that of the 

 other form from near Colorado Springs, which, as before remarked, is 

 essentially like that of Mactra. 



I have not been able to ascertain with precision the geological age of 

 the stratum from which the last-named forms were obtained, but if the 

 shells belong, as I suppose, to the Mactridw, we must, in this case, neces- 

 sarily assume them to be of not later date than the Fox Hills Creta- 

 ceous Group, because that group embraces the last of the true marine 

 deposits in that part of the continent. Eesting immediately upon the 

 Fox Hills Group is the Laramie Group, all the known faunal remains of 

 which are of brackish- water, fresh- water, or land habitat, which of course 

 embrace no Mactridce. Above the Laramie Group all the deposits are 

 of fresh- water origin. 



As to the strata from which the typical examples of Cyrena f holmesi 

 were obtained, after a careful personal examination of the locality, I 

 have no doubt that they really belong to the Fox Hills Group,* and that 

 they have, at that point, been completely inverted by the great outflow 

 of trap, a large part of which still rests on the neighboring Table Mount- 

 ain, and the great vent of which is clearly traceable immediately adja- 

 cent to the fossil locality. 



* This view is still further strengthened by the discovery, by Mr. Arthur Lakes, of 

 Golden City, of an unmistakable fragment of a Scaphites in the same stratum with the 

 shells here described. 



Since the foregoing portion of this note was written, Mr. Lakes has sent to the Sur- 

 vey a box of fossils from Bear Creek, near Morrison, Colo., among which is a mass of 

 calcareous stone from "750 feet below the coal," charged with shells having all the 

 characteristics of Cyrena? holmesi Meek, and also containing a fragment of a Scapliites 

 (S. mandanensis Morton?), which is evidently identical with the fragment that Mr. 

 Lakes found associated with Cyrena holmesi Meek at the typical locality. This leaves 

 no doubt of the Cretaceous age of the last-named species. 



